System and method for gain control of individual narrowband channels using a wideband power measurement

ABSTRACT

The present invention comprises a system and method for gain control of individual narrowband channels using a wideband power measurement. The present invention comprises a transmit power tracking loop which controls the power of the transmitted signal by controlling the gain applied to the signal. The function of the transmit power tracking loop is to measure power, accept commands for power adjustment, and adjust the power. Gain control is performed by producing an error signal using the comparison of measurements of a total of estimated powers of each narrowband signal and a measurement of the modulated wideband signal. In addition, open loop control is performed by accepting open loop commands generated by an algorithm.

This application is a continuation of U.S. patent application Ser. No.09/150,545, filed Sep. 9, 1998, now U.S. Pat. No. 6,252,915. This patentapplication is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to mobile telephone systems, andmore particularly to a gain control method and system for mobiletelephone systems. The present invention is most applicable to gatewaysusing code division multiple access (CDMA) modulation techniques wherepower conservation is critical.

2. Description of Background Art

Mobile telephone systems allow customers to place telephone calls fromwireless devices referred to as mobile telephones or subscriber units.The mobile telephone transmits the signal to a gateway. The gateway isinterconnected to a mobile telephone switch. The mobile telephone switchinterconnects the gateways to each other and to public switchedtelephone networks (PSTNs).

One method that is used for mobile telephone transmission to a gatewayis via a ground-based antenna that operates in UHF band. This is thesame band used for broadcast television transmission. Use of this methodlimits the subscriber to communication within a cell which is theserving area to which the antenna can transmit using UHF band.Subscribers can move from cell to cell because hand-offs are possiblefrom one cell to another. However, if no ground-based antenna is withina distance that can be reached using UHF band, such as in a rural area,a subscriber cannot use the mobile telephone.

Developments in mobile telephone system technology have led to mobiletelephone systems that can transmit using a low earth orbit (LEO)satellite system, such as the Globalstar LEO satellite system. Themobile telephone systems that use LEO satellite systems can transmit torural areas because the subscriber does not need to be within closerange of a ground-based antenna. As a result, mobile telephone systemsusing LEO satellite systems are not limited to major cities as aremobile telephone systems that use antennas operating in the UHF band.

The transponder is the component in a satellite that receives andtransmits signals from subscribers using mobile telephones. A satellitetransponder must be able to carry a large number of subscriberssimultaneously in order to be cost effective. Various satellite accessschemes such as time division multiplex access (TDMA) and code divisionmultiplex access (CDMA) allow simultaneous access to transponders by alarge number of subscribers.

Digital CDMA is preferable to other satellite access schemes as morecustomers can be carried at a lower cost and higher quality. Low poweredsignals allow transmission of CDMA signals via small, inexpensiveantennas requiring less expensive earth station and network equipmentthan other satellite access systems. However, because signal power islow, the power must be used efficiently. CDMA systems have low noise andinterference because the gateways transmit using low powered signals.

In a CDMA system each customer is carried on an individual channel. CDMAsystems modulate and interleave the individual channels so that a largenumber of channels are spread throughout the same waveform. As a result,multiple customers or users simultaneously share the same subbeam whichis referred to interchangeably herein as a narrowband channel or acarrier. A subbeam or narrowband channel is typically approximately 1.23MHZ in bandwidth.

Because multiple customers or users share the same subbeam, if onecustomer's or user's signal is transmitted at a higher power than thesignals of the other customers or users on the channel, interference mayoccur which may result in unacceptable performance unless the number ofusers on the subbeam is reduced. In addition, lower power transmissionhelps overcome fading because signals can be spread through more of thesubbeam and more capacity is available in the subbeam for diverse paths.Also, lower power transmission conserves power at the gateway. However,if the power of a customer's signal becomes too low, the quality ofservice for that customer becomes unacceptable.

For transmission via satellite, individual subbeams are modulatedtogether to create one wideband channel. A wideband channel comprises104 subbeams and has a bandwidth of 160 MHZ. However, the number ofsubbeams that may be carried by a wideband channel's ability isdependent on the power of each subbeam. The power available to transmituser traffic is the power that the satellite is capable of transmittingless the overhead power required for satellite operation. The number ofusers that may be transmitted essentially equals the power available totransmit user traffic divided by the power required for each individualuser. Thus, the number of users that may be provided service isincreased by maintaining overhead power levels and each individualuser's channel at the minimum levels needed for optimum performance.This can be accomplished by limiting the power of each subbeam to thepower necessary for high quality transmission. Control of the power ofthe subbeams and the wideband channel is needed to limit the power ofeach subbeam and wideband channel to the power needed for high qualitytransmission and to ensure efficient use of power which allows themaximum number of subbeams, and individual channels, to be carried on awideband channel.

In addition, an accurate accounting of the power being consumed on thesatellite is needed to manage the health of the satellite and reducecosts of satellite transmission. If power demands on a satellite exceeddesign expectations, satellite batteries will be relied on as energysources more often than planned during satellite design. Additionaldemands on satellite batteries will require that the batteries bereplaced more often at a cost to the satellite service provider.Satellite service providers often charge for the satellite powerconsumed. As a result, satellite service consumers need to trackaccurate measurements of their power demands on the satellite tomaintain the power demands to the minimum level needed to providequality service in order to reduce costs.

SUMMARY OF THE INVENTION

The present invention is a novel and improved method for the control ofthe gain of individual narrowband channels using a wideband powermeasurement. The system of the present invention is a transmit powertracking loop which controls the power by adjusting the gain applied tothe transmitted signal.

Gain control by the transmit power tracking loop provides modificationsin the gain applied by a variable gain amplifier based on feedback ofpower measurements before and after the signal is amplified by thevariable gain amplifier. The power measurements include estimations ofthe power of each individual subbeam prior to amplification and anestimation of the power of the wideband channel after amplification. Thepower estimates of each individual subbeam are summed for the comparisonwith the power of the wideband channel.

Open loop gain control is also provided by controlling gain applied bythe variable gain amplifier using open loop commands. Open loop commandsare generated by an algorithm that calculates the proper gain thatshould be applied by the variable gain amplifier at regular intervals,such as one second. The algorithm for generating open loop commandscalculates the gain that should be applied using the elevation of theantenna, the gain of the dish of the antenna, and various constants.Open loop commands are implemented by adjustments in gain applied by thevariable gain amplifier and adjustments of gain applied by eachsubbeam's individual modulator.

In order to maintain an accurate accounting of the power used by thesatellite, power levels at the gateway are stored and used to provideestimations of the power used by the satellite. The power estimates ofeach individual subbeam and of the wideband channel are sent to a groundoperations control center to be stored for use by other processes toprovide analysis, such as of the power consumed by the satellite. Thegateway power measurements enable the satellite service consumer toobtain an accurate accounting of the power being consumed on thesatellite. An accurate accounting of the power being consumed on thesatellite enables the satellite service consumer to maintain the powerdemands at the minimum level needed to provide quality service andreduce costs. In addition, satellite service providers may use thisinformation to manage the health of the satellite and reduce costs ofsatellite transmission.

Gain control balances the need to transmit at a low power to utilizecapacity for as many customers as possible, avoid overdriving thesatellite, and avoid violating flux density limits while maintainingsufficient power for each subbeam to provide high quality service to theusers carried on the subbeam. Gain control also allows for adjustment toconditions at the gateway.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like reference numbersindicate identical or functionally similar elements. Additionally, theleft-most digit of a reference number identifies the drawing in whichthe reference number first appears.

FIG. 1 is a block diagram of a mobile telephone system environmentaccording to a preferred embodiment of the present invention;

FIG. 2 is a block diagram of gateway transmit equipment including atransmit power tracking loop according to a preferred embodiment of thepresent invention;

FIG. 3 is a flowchart illustrating the operation of gain adjustment of atransmit power tracking loop according to a preferred embodiment of thepresent invention;

FIG. 4 is a flowchart illustrating the operation of the transmit powertracking loop open loop control for setting GMOD RF values according toa preferred embodiment of the present invention;

FIG. 5 is a flowchart illustrating the operation of the transmit powertracking loop open loop control for setting variable gain amplifier gainaccording to a preferred embodiment of the present invention; and

FIG. 6 is a block diagram of a transmit power tracking loop input-outputmodel.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned previously, the present invention comprises a system andmethod for the control of the gain of individual subbeams or narrowbandchannels using a wideband power measurement. One embodiment of thepresent invention is in a mobile telephone system that uses LEOsatellites for transmission and CDMA technology for satellite access.

A satellite based mobile telephone system can best be described byreferencing the processing of a typical call. FIG. 1 is a block diagramof a satellite based mobile telephone system environment. A mobiletelephone system 104 which communicates with other networks in thesatellite based mobile telephone system environment comprises thecomponents related to satellite based mobile telephone service. Thecomponents for a satellite based mobile telephone service are mobiletelephones 106A, 106B, . . . 106 n, low earth orbit satellites 108A,108B, . . . 108 n, antennas 112A, 112B, . . . 112 n, gateways 110A, 110B. . . 110 n, digital links 114A, 114B, . . . 114 n, and mobile telephoneswitches 118A, 118B, . . . 118 n and ground operation control centers117A, 117B, . . . 117 n located in mobile telephone switching offices116A, 116B, . . . 116 n. The total number of mobile telephone switches118, gateways 110, satellites 108 and other equipment in a mobiletelephone system 104 depend on desired system capacity and other factorswell understood in the art.

An exemplary call can be described by referencing one of the pluralityof each of the components illustrated in FIG. 1 that would be used tocarry a particular call. A subscriber may place or receive a call usinga mobile telephone 106. Other devices that may be used to place orreceive a call include a data transceiver, a paging or positiondetermination receiver, a wireless personal computer, and any otherdevice that communicates via a wireless telecommunication network. Alarge number of subscribers may place or receive calls simultaneously.Each individual mobile telephone 106 is a wireless unit that includes akeypad, an earpiece, and a mouthpiece. Each mobile telephone 106communicates directly with a satellite 108.

Satellites 108A, 108B, . . . , 108 n together comprise a LEO satellitesystem. One such planned LEO satellite system includes 48 satellitestraveling in low earth orbits approximately 763 miles from the earth'ssurface and inclined 50 degrees from the equator. The present inventioncould be used with other satellite communications, including satellitesystems located at other distances and orbits, and withterrestrial-based wireless systems where power maintenance is aconsideration.

Antenna 112 receives and transmits signals from and to mobile unit 106via the satellite 108. Antenna 112 sends the signal received from themobile unit to the gateway 110. If a mobile telephone is receiving acall or signal with information, antenna 112 receives the signal fromgateway 110, as will be described in further detail with reference toFIG. 2, and transmits the signal to mobile unit 106 via satellite 108.Gateway 110 is connected to mobile telephone switch 118 located inmobile telephone switching office 116 via digital link 114, which isalso referred to as a land line. Land lines are wired telecommunicationlinks, such as copper or fiber optic cables.

The mobile telephone switch 118 is located in a mobile telephoneswitching office 116 which also houses other equipment needed to processmobile telephone calls. Mobile telephone switch 118 interconnectsgateways 110 to each other and to public switched telephone network(PSTN) 120, as shown in FIG. 1.

Ground operation control centers 117 also within the mobile telephoneswitching centers 116 control the equipment within the mobile telephonesystem 104. Ground operation control center 117 controls mobiletelephone 106, satellite 108, antenna 112, gateway 110, and mobiletelephone switch 118 in the processing of the exemplary call. The groundcontrol centers 117 are connected to the mobile telephone switch 118that are located within the same mobile telephone switching center 116.In addition, the ground control centers 117 are connected to the digitallinks 114 in order to control equipment that is not located in themobile telephone switch centers 116. The ground operation controlcenters 117 control the power of the signals transmitted by theequipment of the mobile telephone system 104 in order to confine thepower to the levels needed for successful transmission and conservepower to maximize the number of calls that can be transmitted.

The PSTN 120 accepts and terminates calls to telephones 122A, 122B, . .. 122 n. The PSTN 120 can accept and terminate calls to any device thatcan communicate via a telecommunications network such as facsimilemachines and personal computers.

FIG. 2 is a block diagram of gateway transmit equipment including atransmit power tracking loop 202 in gateway 110. The gateway 110modulates and transmits signals from the mobile telephone switchingoffice 116 to the mobile telephone 106 via the ground-based antenna 112and satellite 108. One embodiment of the system and method of gaincontrol of the present invention is implemented to control the gain of asignal being transmitted by the gateway transmit equipment 202. Thegateway transmit equipment 202 modulates and transmits signals from themobile telephone switching office 116 to the ground-based antenna 112.In addition to transmitting signals, the gateway 110 receives anddemodulates signals from the ground-based antenna 112 to the mobiletelephone switching office 116.

The gateway transmit equipment 202 comprises forward link equipmentwhich transmits the signals to the mobile telephone 106 via thesatellite 108. The transmit power tracking loop is implemented in theforward link system that transmits signals from the antenna 112 at thegateway 110 to the mobile telephone 106. The forward link refers to asegment of the system that sends forward signals from the gateway 110 tothe mobile telephone 106. The reverse link refers to a segment of thesystem that accepts reverse signals from the mobile telephone 106 to thegateway 110. Power is controlled in the forward link system by adjustingthe gain of a variable gain amplifier 212. This controls the power ofthe signal transmitted by the satellite transponder because thesatellite transponder transmits a signal at the same power as the signalwas received by the transponder.

The forward link equipment includes modulators 204A, 204B, . . . 204 n,upconverters 206A, 206B, . . . 206 n, a subbeam summer 208, powerestimators 210A, 210B, . . . 210 n, an amplifier 212, analog hardware214, and antenna 112. The modulators are interconnected to the mobiletelephone switching office 116 via digital link 114. Each of themodulators 204 is connected to a corresponding one of the upconverters206 for upconverting the modulated signal from infrared band frequency(IF) to radio band frequency (RF). The upconverters 206 are connected toan input of subbeam summer 208 which sums the signals. Each of themodulators is also connected to digital link 114 to receive open loopcommands for gain adjustment applied by the modulators 204. Thealgorithm for determining gain adjustment applied by the modulators 204will be discussed in further detail with respect to FIG. 4.

An output of subbeam summer 208 is connected to amplifier 212 whichamplifies the signal. Amplifier 212 is connected to analog hardware 214which performs additional conversions for satellite transmission. Analoghardware 214 is connected to the ground-based antenna 112 whichtransmits to satellite 108. Satellite 108 transmits the modulated signalto mobile unit 106. Power estimators 210 are connected to the outputs ofeach of the modulators 204 to obtain an estimation of the power of eachindividual subbeam prior to conversion by upconverters 206. Powerestimators 210 are also connected to digital link 114 to provide thepower estimates for each of the individual subbeams to the groundoperations control center 117 for use by other processes.

The gateway transmit equipment 202 also comprises a transmit powertracking loop 216 which allows for gain adjustment using powermeasurements obtained before and after the signal is amplified byvariable gain amplifier 212 and open loop commands. Transmit powertracking loop 216 controls the power of the transmitted signal bycontrolling the gain applied to the signal by variable gain amplifier212 and modulators 204. Transmit power tracking loop 216 includes anestimator summer 218, a logarithm converter 220, an estimate filter 222,a first transmit power tracking loop summer 224, an open loop commandgenerator 226, an error signal digital filter 228, a second transmitpower tracking loop summer 230, a digital to analog (D/A) converter 232,an output power measurement digital filter 234, and a wideband powermeter 236.

An overview of the interconnections of the components of the transmitpower tracking loop 216 follows. Estimator summer 218 is connected toeach of the estimators 210 to sum the estimations of power. Estimatorsummer 218 is also connected to logarithm converter 220 to performconversion to decibel units. Logarithm converter 220 is connected to theestimate filter 222 for filtering. The estimate filter 222 is connectedto the first transmit power tracking loop summer 224 as is the open loopcommand generator 226 and the output power measurement digital filter234. The output power measurement digital filter 234 is connected towideband power meter 236 for filtering the wideband power measurementprior to summing. The first transmit power tracking loop summer 224 isconnected to the error signal digital filter 228 to provide the sum tovariable gain amplifier 212. The error signal digital filter 228 isconnected to second transmit power tracking loop summer 230 as is theopen loop command generator 226 for summing. The second transmit powertracking loop summer 230 is connected to D/A converter 232 for digitalto analog conversion prior to providing the error signal to the variablegain amplifier 212. Wideband power meter 236 is connected to digitallink 114 to send wideband power measurements to ground operationscontrol center 117. Open loop command generator 226 is connected todigital link 114 to obtain information from ground operations controlcenter 117 for determining the open loop commands.

The functions of the components of the gateway transmit equipment willnow be described. Mobile telephone switching office 116 sends the signalto modulator 204. Modulator 204 spread spectrum modulates the voicechannel data and sends the modulated signal to upconverter 216.Modulator 204 is described in further detail in U.S. Pat. No. 5,103,459,entitled “System and Method for Generating Signal Waveforms in a CDMACellular Telephone System” incorporated by reference herein.

Modulator 204 sends the signal to upconverter 206. Upconverter 206converts the frequency from intermediate band frequency (IF) to thehigher radio frequency band (RF). LEO satellites 108 transmit signals atfrequencies in the higher radio band frequency (RF) range. Signalsproduced by modulators 204 are in the intermediate frequency band (IF)range. The conversion performed by the upconverters 206 allows the radiofrequency band (RF) signals from upconverters 206 to be sent to theantenna 112. Upconverters 206 are connected to subbeam summer 208.Subbeam summer 208 adds the signals to obtain a wideband signal. Thewideband signal is sent from subbeam summer 208 to amplifier 212.

Amplifier 212 adjusts the gain based on gain adjustment using powermeasurements obtained before and after the signal is amplified byvariable gain amplifier 212 and open loop commands. In addition,modulator 204 adjusts gain of individual subbeams. Determination of gainadjustments will be discussed in further detail with respect to FIGS.3-5.

Amplifier 212 sends the signal to analog hardware 214 for additionalconversions needed for satellite transmission. The resulting modulatedsignal is sent to antenna 112 for transmission to the mobile unit.

Transmit power tracking loop 216 receives input from two sources. Onesource is estimated subbeam power measurements determined by powerestimators 210. In order to obtain the estimated power measurements frommodulators 204, the output of each of the modulators 204 is connected toa corresponding one of the power estimators 210. Power estimators 210provide an estimation of the power of each of the individual subbeams.Power estimators 210 are connected to estimator summer 218 whichprovides a sum of the estimated power output from modulators 204. Powerestimators 210 are also connected to digital link 114 to provide the sumof the estimated power to ground operations control center 117.

The second source for transmit power tracking loop 216 is a powermeasurement of the signal output from analog hardware 214 obtained bywideband power meter 236. Wideband power meter 236 is connected to theoutput of analog hardware 214 to obtain the measurement of the power ofthe signal output from the analog hardware 214 that is transmitted byantenna 112. Various wideband power meters suitable for this purpose areavailable on the market. One such meter is an HP437B wideband powermeter.

The power measurements obtained by estimators 210 are summed byestimator summer 218. As noted earlier, a wideband channel comprises 104individual subbeams. Therefore, estimator summer 218 provides the totalestimated power from 104 estimators 210.

Estimator summer 218 provides a summation of the outputs to a logarithmconverter 220. The logarithm converter 220 determines the logarithm ofthe summation of the outputs and multiples the logarithm by 10. Thisconverts the summation of the outputs to decibel units.

Logarithm converter 220 provides the summation of the output in decibelunits to estimate filter 222. A filter reduces or eliminates power atspecific frequencies. The estimate filter 222 is implemented insoftware, therefore, the design of the estimate filter 222 can bechanged by changing the code of the software. However, the estimatefilter 222 is not limited to a software implementation and may beimplemented using any filter component. The estimate filter 222 is acomposite of a filter used in estimating the power on an individualsubbeam basis and a filter on the total estimated power of theindividual subbeams. The gain of the estimate filter 222 is modeled inthe transmit power tracking loop input-output model as the estimatefilter gain 610 shown in FIG. 6. The estimate filter 222 provides thefiltered summation of the output in decibel units to a first transmitpower tracking loop summer 224.

Open loop command generator 226 also is connected to the first transmitpower tracking loop summer 224. Open loop command generator 226 providesopen loop commands 226 to variable gain amplifier 212 via the transmitpower tracking loop 216. The open loop commands cause an adjustment inthe gain applied to the signal by the modulator 204 and the variablegain amplifier 212. The process for generating open loop commands isdescribed in more detail with respect to the transmit power trackingloop open loop procedures illustrated in FIGS. 4 and 5.

An output power measurement digital filter 234 is also connected to thefirst transmit power tracking loop summer 224. Similar to the estimatefilter 222, the output power measurement digital filter 234 isimplemented in software, therefore, the design of the output powermeasurement digital filter 234 can be changed by changing the code ofthe software. Also, similar to the estimate filter 222, the output powermeasurement digital filter 234 is not limited to a softwareimplementation and may be implemented using any filter component. Theoutput power measurement digital filter 234 is a composite filter thatis applied to the signal that is output from the wideband power meter236. The output power measurement digital filter 234 is a composite ofboth an analog and digital filter that reduce or eliminate power atspecific frequencies from the signal output from the wideband powermeter 236. The analog filter is internal to the wideband power meter236. The gain of the analog filter is represented as output powermeasurement analog filter gain 622 in the transmit power tracking loopinput-output model 602 illustrated in FIG. 6. The digital filter is thecascade of the digital filter in the wideband power meter 236 and thedigital filter in gain control unit. The digital filter is representedas the output power measurement digital filter gain 626 in the transmitpower tracking loop input-output model 602 illustrated in FIG. 6.

Wideband power meter 236 measures the power of the signal afteramplification by amplifier 212. This is also after the individualsubbeams have been summed into one wideband channel by subbeam summer208. As a result, the power measured by wideband power meter 236 is thepower of the wideband channel.

The response of wideband power meter 236 used to measure the transmittedpower is fundamental to transmit power tracking loop 216. In oneembodiment, wideband power meter 236 samples the power sensor voltageevery 50 ms. In other words, the sampling rate is 20 Hz.

First transmit power tracking loop summer 224 receives the filteredsummation of the estimated powers of each subbeam from the estimatefilter 222, the open loop commands from open loop command generator 226,and the filtered wideband power measurement from the output powermeasurement digital filter 234. The first transmit power tracking loopsummer 224 sums the output from the estimate filter 222, the output ofthe open loop command generator 226, and the negative of the outputpower measurement digital filter 234. In other words, the output fromfirst transmit power tracking loop summer 224 is the sum of the outputof the estimate filter 222 and the output of the open loop commandgenerator 226 less the output from the output power measurement digitalfilter 234. The value of the output from the first transmit powertracking loop summer 224 is also referred to as the error signal becauseit provides error information for the adjustment in gain applied by thevariable gain amplifier 212 to compensate for errors such as noise andother imperfections in the hardware of the system.

The output from the first transmit power tracking loop summer 224 isprovided to the error signal digital filter 228. Similar to the estimatefilter 222 and the output power measurement digital filter 234, theerror signal digital filter 228 is implemented in software, therefore,the design of the error signal digital filter 228 can be changed bychanging the code of the software. However, as with the other filters,the error signal digital filter 228 is not limited to a softwareimplementation and may be implemented using any filter component. Theerror signal digital filter 228 reduces or eliminates power a specifiedfrequencies from the output of the error signal.

As discussed with respect to FIG. 6, the estimate filter 222 and theoutput power measurement digital filter 234 are designed to have thesame gain resulting in zero difference output from the first transmitpower tracking loop summer 224. Therefore, the error signal digitalfilter 228 which is applied directly to the error signal is the primarycomponent responsible for the performance of the transmit power trackingloop 216.

The output of the error signal digital filter 228 is provided to thesecond transmit power tracking loop summer 230. The second transmitpower tracking loop summer 230 also receives the open loop commandsgenerated by the open loop command generator 226. The second transmitpower tracking loop summer 230 sums the output from the error signaldigital filter 228 and open loop command generator 226.

The output of the second transmit power tracking loop summer 230 is sentto D/A converter 232. D/A converter 232 converts the digital signalreceived from the second transmit power tracking loop summer 230 to ananalog signal. D/A converter 232 performs the digital to analogconversion by taking many samples of the discrete instructions in ashort period of time and creating an analog waveform. D/A converter 232is connected to variable gain amplifier 212 to provide the analog errorsignal to the variable gain amplifier 212 to allow variable gainamplifier 212 to apply the correct gain to the signal sent to antenna112.

Transmit power tracking loop 216 is calibrated before the powermeasurements of the signal sent to satellite 112 are obtained. Powerestimators 210 provide power estimates of the individual narrowbandchannels received from each of the modulators 204 prior toamplification. Unfortunately, power estimators 210 add noise to thesignal. In addition, the signals output from power estimators 210 arefiltered. To ensure precision, the components of transmit power trackingloop 216 are selected based on a model that includes the effects of thenoise and filter. The model will be discussed in further detail in FIG.6.

The transmit power tracking loop 216 may be implemented using computersoftware which is stored on an application specific integrated circuit.However, the present invention may be stored on any one or more computercomponents that are capable of processing computer software. Inaddition, the present invention is limited to a software implementationand may be implemented on any electronic components that perform thefunctions described.

FIG. 3 is a flowchart 302 which illustrates the operation of gainadjustment of transmit power tracking loop 216. The gain adjustmentallows for modifications in the gain applied by variable gain amplifier212 based feedback of power measurements before and after the signal isamplified by variable gain amplifier 212.

In step 306, transmit power tracking loop 216 performs a softwarecalibration. The gain adjustment performed by transmit power trackingloop 216 is based on the gain calculated from the sum of the estimatedpowers of the narrowband channels and a measurement of the power of thewideband signal immediately prior to transmission via antenna 112.

In the simplest case, the gain of the individual narrowband channels isset to be equal. The gain, referred to as G_(i) in the equations below,is the power of the individual narrowband channel that is output fromamplifier 212 divided by the power of the individual narrowband channelthat is output from modulator 204, which is referred to as P_(i).Therefore, the power of the individual narrowband channel that is outputfrom amplifier 212 divided by the power of the individual narrowbandchannel that is output from modulator 204 is the same for eachindividual narrowband channel.

Calibration is performed prior to the satellite coming into view.Calibration is performed by putting out a reference level of poweroutput from each of the modulators 204. Therefore, P_(i) is the same foreach of the individual subbeams during calibration. Next the power ofeach individual narrowband channel output from amplifier 212 ismeasured. The ratio of the powers is set to a certain number whichcalibrates each individual narrowband channel.

In another embodiment, the gain, G_(i), of each individual subbeam on awideband channel does not need to be equal. Error correction software inmodulators 204 can adjust the gain of individual channels to compensatefor the effect of amplifier 212. The assumption used in determiningadjustment to the gain is based on the power of each of the subbeamsfrom the modulators 204 being equal. If the power of each of theindividual subbeams is not equal, a subbeam's power can be adjusted bythe corresponding modulator 204 that performs the modulation for theparticular subbeam. Adjustment by the individual modulator 204 is alsoreferred to as error correction. Error correction is based on the actualpower measured out of the analog hardware 214 and the gain appliedduring the calibration procedure. Error correction can be implemented bycommands from ground operations control centers 117 in the same way thatmodulators 204 accept open loop commands. Setting modulator gain withopen loop commands will be discussed in further detail with respect toFIG. 4.

In step 308, the transmit power tracking loop 216 estimates the power ofthe individual narrowband channels. The power of an individualnarrowband channel is represented in the following equations as P_(i).As mentioned previously, the components of the transmit power trackingloop 216 that provide power estimates of the individual narrowbandchannels are the power estimators 210. Each narrowband channel has acorresponding estimator 210 that provides a power estimate for thatparticular narrowband channel.

In step 310, the estimated powers of the individual narrowband channelsare added for a total estimated power of the wideband channel. Theestimator summer 218 provides the summation of the outputs of the 104power estimators 210 which is the sum of the estimated powers of theindividual narrowband channels. The sum of the estimated powers of thenarrowband channels is represented as P_(SOFT) in Equation (1) below.$\begin{matrix}{P_{SOFT} = {\sum\limits_{i = 1}^{n}P_{i}}} & (1)\end{matrix}$

In step 312, the transmit power tracking loop 216 obtains a measurementof the output power of the wideband channel after amplification. Theoutput power of the wideband channel after amplification is measured bywideband power meter 236. This output power measurement is representedby P_(GRS) in the equations below. If the system were free of errors,the output power measurement P_(GRS) would be equal to the summation ofthe amplified estimated powers as represented in Equation (2) below.$\begin{matrix}{P_{GRS} = {\sum\limits_{i = 1}^{n}{P_{i}*G_{1}}}} & (2)\end{matrix}$

In step 314, the transmit power tracking loop 216 compares the outputpower obtained by wideband power meter 236 and the sum of the amplifiedestimated powers. Because a software calibration is performed in step310, the gain for each of the individual subbeams can be treated asbeing equal. This is reflected in Equation (3) below. $\begin{matrix}{P_{GRS} = {G*{\sum\limits_{i = 1}^{n}P_{i}}}} & (3)\end{matrix}$

As a result, the gain for the overall system can be determined bycomparing the output power measured by wideband power meter 236 and thesum of the estimated powers provided by the estimation summer 218.Equation (4) below provides the calculation for overall system gain.

G=P _(GRS) /P _(SOFT)  (4)

In step 316, the transmit power tracking loop adjusts the gain of theindividual subbeams using variable gain amplifier 212.

FIG. 4 is a flowchart 402 which illustrates the transmit power trackingloop open loop procedure for setting GMOD RF gain values. The transmitpower tracking loop open loop procedure for setting GMOD RF gain valuesis the algorithm used to determine the open loop commands sent to themodulators 204 for adjustment of gain of each individual subbeam.

In one embodiment, the transmit power tracking loop open loop procedureis performed every second. The time interval in which the transmit powertracking loop open loop procedure is being performed is referred to astime interval k. The transmit power tracking loop open loop procedure isperformed for each active subbeam within each active beam. The subbeamfor which the transmit power tracking loop open loop procedure is beingperformed in time interval k is pointed to in software by pointer j andmay be referred to as subbeam j. The beam that includes the subbeam j ispointed to in software by pointer i and may be referred to as beam i.

In step 406, the transponder gain for each beam i and subbeam j(G_TRANS_i_j(k)) is retrieved. The transponder gain adjustment for beami and sunbeam j during transmit power tracking loop time step k is readfrom a table generated by a pre-contact gain calculation procedure. Thepre-contact gain calculation procedure is performed prior to thetransmit power tracking loop procedure to obtain values of the dataneeded by the transmit power tracking loop procedure.

In step 408, the antenna elevation (THETA_GW(k)) is retrieved. Thetransmit power tracking loop open loop procedure requires the elevationof antenna 112 to be calculated every time step, which is 1 second. Thetransmit power tracking loop open loop procedure retrieves the value ofTHETA_GW(k) from memory within ground operations control center 117 viadigital link 114. The value of THETA_GW(k) was stored in memory ofground operations control center 117 as the result of processing ofanother software procedure. The value of the gateway antenna elevationis used to calculate the path gain (G_PATH(k)).

In step 410, the path gain for the subbeam (G_PATH_i_j(k)) iscalculated. G_PATH_i_j(k) is obtained from a look-up table of G_PATH_iversus antenna 112 elevation. THETA_GW(k) is used to read from thetable. Different tables are used for different polarizations. However,subbeams within a beam use a common table. The value of G_PATH_i_j(k) iscalculated using linear approximation between points in the table. Thevalues within the table are in decibels and increase with antenna 112elevation. The increase is from 0E to 90E with a uniform step size of0.5°.

In step 412, the modulator 204 (GMOD) to radio frequency (RF) hardware236 gain (GMOD RF gain) is calculated. The GMOD RF gain value iscalculated from Equation 5 below.

G _(—) RF _(—) i _(—) j(k)=−(G _(—) PATH _(—) i _(—) j(k)+G _(—) TRANS_(—) i _(—) j(k))   (5)

In step 414, the modulator 204 (GMOD) to radio frequency (RF) hardware236 gain (GMOD RF gain) for each individual subbeam is calculated fromEquation 6 below. $\begin{matrix}{{g\quad \_ \quad {RF}\quad \_ \quad i\quad \_ \quad {j(k)}} = {{round}\quad \left( {g\quad \_ \quad {RF}\quad \_ \quad {o \cdot \left( {10\frac{G\quad \_ \quad {RF}\quad \_ \quad i\quad \_ \quad {j(k)}}{20}} \right)}} \right)}} & (6)\end{matrix}$

In Equation 6 above, g_RF_o is the nominal GMOD RF gain value which inthe preferred embodiment is 46. The value of g_RF_i_j(k) is the GMOD RFgain for beam i and subbeam j in linear units. The parentheses indicatethat the value should be rounded to the nearest integer. The value ofg_RF_i_j(k) is restricted to the range 33 to 63. Values calculatedoutside of this range should be truncated. The value of g_RF_i_j(k) iscalculated for each active subbeam during time step k.

In step 416, a determination is made as to whether all frequencies withactive subbeams have been analyzed using the transmit power trackingloop open loop procedure. If in step 416 it is determined that allfrequencies with active subbeams have not been analyzed using thetransmit power tracking loop open loop procedure, then the transmitpower tracking loop procedure proceeds to step 418. If in step 416 it isdetermined that all frequencies with active subbeams have been analyzedusing the transmit power tracking loop open loop procedure, then thetransmit power tracking loop procedure proceeds to step 420.

In step 418, the transmit power tracking loop open loop procedureincrements the subbeam pointer j to point to the next subbeam. If all ofthe subbeams in a beam have been analyzed, the transmit power trackingloop open loop procedure increments the beam pointer i to point to thenext beam and the subbeam pointer j to point to the first subbeam withinthe next beam.

In step 420, the transmit power tracking loop open loop procedure setsthe active GMOD RF gain values with g_RF_i_j(k). The GMOD RF gain valuefor a particular modulator 204 is updated only if the value ofg_RF_i_j(k) is different from g_RF_i_j(k!1).

FIG. 5 is a flowchart 502 which illustrates the operation of thetransmit power tracking loop open loop procedure for setting thevariable gain amplifier 212. The transmit power tracking loop open loopprocedure for setting the variable gain amplifier 212 is the algorithmused for determining open loop commands to be sent to the variable gainamplifier 212 for gain adjustment of the wideband channel.

In step 506, the antenna gain (THETA_GW(k)) is obtained. The transmitpower tracking loop open loop procedure retrieves the value ofTHETA_GW(k) from memory within ground operations control center 117 viadigital link 114. The value of THETA_GW(k) was stored in memory of theground operations control center 117 as the result of processing ofanother software procedure.

In step 508, the path gain common to all beams and subbeams (G_PATH(k))is calculated. G_PATH(k) is calculated using linear interpolationbetween points in a table of G_PATH versus antenna 112 elevationTHETA_GW(k). Because the antenna 112 is a C-band antenna pattern whichis rotationally symmetric, the path gain is only a function of theantenna 112 elevation. The table used to calculate G_PATH(k) is based ona curve that is plotted using assumptions that minimize the peak-peakdeviation of the curve at various C-band frequencies. Therefore, thecurve does not correspond to the path gain at the mid-band or any otherfrequency. This is done to conserve GMOD RF gain adjustment range.

Table 1 shows the structure of the G_PATH versus THETA_GW table. Note,the gateway antenna elevation is monotonically increasing from 0° to 90°with a uniform step size of 0.5°.

TABLE 1 G_PATH versus THETA_GW Table Structure Index THETA_GW (Degrees)G_PATH (dB)  0 0 −176.430  1 0.5 −176.291  2 1.0 −176.152 . . . . . . .. . 179 89.5 −168.016 180 90 −168.02

In step 510, the transponder gain (G_TRANS(k)) is retrieved. Apre-contact gain calculation procedure stores the value of the commontransponder gain in 1 second intervals. Every transmit power trackingloop time step, the value of k is used to retrieve a value forG_TRANS(k) stored during the pre-contact gain calculation procedure.

In step 512, the antenna dish gain (G_DISH) is retrieved. During thepre-contact gain calculation procedure, a value of G_DISH is alsodetermined. The value of G_DISH is valid for all time steps during thepass.

G_DISH is defined as the arithmetic mean of the minimum antenna 112 gainand the maximum antenna 112 gain taken from the ensemble formed by theantenna gains of all of the possible subbeam frequencies in apolarization. The antenna gain includes the gain of the feed networkstarting from the power meter test coupler. A table of G_DISH versuspolarization stores the values of G_DISH. One value is stored for eachpolarization for a total of eight values. The information stored in thetable is shown in Table 2.

TABLE 2 Center Gateway Antenna Gain Versus Polarization AntennaPolarization G_DISH 0 LHCP 48 dB (to be provided by antenna vendor) 0RHCP 48 dB (to be provided by antenna vendor) 1 LHCP 48 dB (to beprovided by antenna vendor) 1 RHCP 48 dB (to be provided by antennavendor) 2 LHCP 48 dB (to be provided by antenna vendor) 2 RHCP 48 dB (tobe provided by antenna vendor) 3 LHCP 48 dB (to be provided by antennavendor) 3 RHCP 48 dB (to be provided by antenna vendor)

In step 514, the gateway common gain (G_GW(k)) is calculated. TheGateway gain common to all beams and subbeams for time step k iscalculated from Equations (7)-(10) below.

G _(—) GW(k)=G−(G _(—) RF _(—) o+G _(—) FIXED+G _(—) DISH+G _(—)PATH(k)+G _(—) TRANS(k))  (7) $\begin{matrix}{G = {54.2323\frac{{BW}}{{BLSB}^{2}}}} & (8)\end{matrix}$

 G _(—) RF _(—) o× 20 log (46)=33.25 dBLSB ²  (9)

G _(—) FIXED=−60.096 dB  (10)

In step 516, the variable gain amplifier gain is set using G_GW(k).

FIG. 6 illustrates a transmit power tracking loop input and output model602. The inputs and output are defined in Table 3 below. FIGS. 2 and 3will be referenced to assist in the description. The transmit powertracking loop input and output model uses wideband measurements. Inaddition, the modeling is done based on the assumption that thecalibration procedure has been instituted.

TABLE 3 Transmit Power Tracking Loop Inputs and Output A Estimate filtergain 610 B Output Power Measurement Composite Filter Gain (not shown inFIG. 6) B_(s) Output Power Measurement Analog Filter Gain 622 B_(z)Output power measurement digital filter gain 626 C Command input filtergain 614 D VGA response 620 E Estimate Noise 608 G Command Gain 612G_(o) Hardware imperfection gain 606 GAIN Transmit Power Tracking LoopGain 628 K Error signal filter gain 616 N Output Power Measurement Noise624 P_(m) Transmit Power Tracking Loop Input Power 604 P_(o) OutputPower 630 Z D/A Response 618

The input in the transmit power tracking loop input and output model 602is the transmit power tracking loop input power 604 referred to in theequations below and Table 1 above as P_(in). The transmit power trackingloop input power (P_(in)) 604 is the power of the sum of the signalsthat are received from the modulators 204 referred to as P_(SOFT) above.P_(SOFT) is determined by the estimator summer 218 by summing theestimated powers of the individual narrowband channels that is obtainedfrom the power estimators 210.

The signal into the transmit power tracking loop 216 has a nominal gainreferred to as the hardware imperfection gain 606 referred to in theequations below and in Table 1 above as G_(o). The hardware imperfectiongain 606 represents a nominal gain selected for modeling purposes torepresent the gain associated with the transmit power tracking loophardware 216.

Another gain associated with the transmit power tracking loop 216 is thecommand gain 612 referred to in Table 1 above and the equations below asG. As discussed, the transmit power tracking loop 210 accepts gainadjustments via open loop commands. The command gain 612 is theamplification of the signal due to open loop commands.

Inaccuracies in the hardware of the transmit power tracking loop causean elevation in the power of the transmitted signals. The first of theseinaccuracies is the estimate noise 608 which is the noise due to theestimate of the input power referred to in the equations below and inTable 1 above as E. The power estimators 210 estimate the input powerinto the transmit power tracking loop 216 as discussed in step 306 ofFIG. 3. The circuitry that a signal passes through, such as circuitry ina power estimator 210, has imperfections that provide a small amount ofpower to the system which is also referred to as noise. The noise causedby circuitry in the estimators 210 is the estimate noise 608.

The second inaccuracy is the noise due to the measurement of outputpower 624 referred to in Table 1 above and the equations below as N.Similar to other components of the transmit power tracking loop 210, thecircuitry of the power meter 236 causes a slight amplification of thesignal which is also referred to as noise. The noise due to the powermeter 236 is referred to as the noise due to the measurement of outputpower 624.

The power inputs to the transmit power tracking loop 210 are filtered byfour filters implemented in software. A filter reduces or eliminatespower at specified frequencies. Because the filters are implemented insoftware, the design of the filters can be changed by changing the codeof the software. Because of imperfections in the circuitry of thesefilters, each of these filters causes a gain in the system.

The first of these filter gains is the estimate filter gain 610 referredto in Table 1 and the equations below as A. The software estimate ofinput power filter gain 610 is a composite of a gain of a filter used inestimating the power on an individual beam basis and a gain of a filteron the total estimated power of the individual subbeams.

The second filter gain is the command input filter gain 614 referred toin Table 1 and the equations below as C. The command input filter gain614 is a filter applied to the open loop commands output from the openloop command generator 226.

The third filter gain is the error signal filter gain 616, referred toin Table 1 and the equations below as K. The error signal filter gain616 digitally filters the error signal that is provided to the variablegain amplifier 212. As discussed with respect to step 316 of FIG. 3,transmit power tracking loop 216 adjusts the gain of the wideband signalusing amplifier 212. The gain is adjusted via an error signal producedby variable gain amplifier 212. The error signal filter gain 616 is theprimary component responsible for the performance of the transmit powertracking loop 216.

The fourth filter is the output power measurement composite filter gainwhich is the composite of the analog and digital filter gains applied tothe measurement of output power. The output power measurement compositefilter gain is referred to in Table 1 above and the equations below as B(and is not shown in FIG. 6). The output power measurement compositefilter is the composite of both an analog and digital filter that areapplied to the signal output from power meter 236. The gain of theanalog filter is represented as an output power measurement analogfilter gain 622, referred to in Table 1 and the equations below asB_(s). The gain of the digital filter is represented as a output powermeasurement digital filter gain 626, referred to in Table 1 and theequations below as B_(z). The overall filtering affect of the filtersapplied to the signal output from the power meter 236 is given as theoutput power measurement composite filter B in Equation (11) below.

B=B _(s) *B _(z)  (11)

The VGA response 620, referred to as D in Table 1 above and theequations below, is the response of variable gain amplifier (VGA) 212 tochanges in the control voltage. Equipment performance varies based onthe voltage used to operate the equipment. The VGA response 620 is ameasurement of the response of variable gain amplifier 212 to thechanges in voltage that provide power to variable gain amplifier 212.The variable gain amplifier 212 is assumed to have a linear gain slopein dB/V so the value is constant.

An additional input is the D/A response 618, referred to in Table 1above and the equations below as Z. The D/A response 618 is the model ofthe response of the D/A converter 232 to discrete instructions. D/Aconverter 232 takes many samples of the discrete instructions in a shortperiod of time and creates an analog waveform. In order to include theresponse of the D/A converter 232 in the model, it is modeled as a zeroorder hold response because each one of the actual samples taken cannotbe shown.

The output power 630, is the power that is output resulting from theinputs described. The output power 630 is the power of the signaltransmitted by antenna 112.

The transmit power tracking loop gain 628, referred to as GAIN in Table1 and the equations below, is the overall gain of the system which isthe power of the signal transmitted by antenna 112 divided by the powerinto upconverters 216.

The design of transmit power tracking loop 216 is analyzed using thetransmit power tracking loop input and output model 602. The transmitpower tracking loop 216 is a multi-input, single output system. Thesingle output is the output power 630 which is the transmit powertracking loop input power 604 amplified by the transmit power trackingloop gain 628. The transmit power tracking loop gain 628 can becalculated from the various inputs as described below.

The transmit power tracking loop gain 628 is a sum of five responses tothe inputs into the system. The first of these responses, the inputresponse, describes the response of the gateway to the transmittedwaveform power. The optimal design is for a small input response whichis accomplished by choosing filters such that the filtering effect ofthe estimate filter gain 610 and the overall effect of the gain of theanalog and digital power meter filters 622 and 624 are as close aspossible. Equation (12) represents the value of the input response. Thevariables in Equation (12) were provided in Table 1 and are described inmore detail with respect to FIG. 6. $\begin{matrix}{{{Input}\quad {Response}} = {\frac{\left( {A - B} \right){ZDK}}{1 + {ZDBK}}*P_{in}}} & (12)\end{matrix}$

For the preferred embodiment of the present invention, the followingvalues for the estimate filter gain 610, the output power measurementanalog filter gain 622, and the output power measurement digital filtergain 626 were selected. These values are software configurable and canbe modified with software changes. The values are continuous which isshown by defining them as functions of s.

A(s)=6.28/(s+6.28)

B _(s)(s)=fast analog filter in HP437B

B _(z)(s)=6.28/(s+6.28)

The values associated with the digital filters can be converted intodiscrete values by substituting s=(z−1)/T, where T is a 50 millisecondtime period. The discrete values for the preferred embodiment of thepresent invention are below.

B _(z)(z)=0.314/(z−0.686)

A(z)=0.314/(z−0.686)

The second response, the disturbance response, describes the response ofthe gateway to changes in the gain of the analog hardware. Thedisturbance response must reject the gain fluctuation spectrum of thegateway hardware. Consequently, the disturbance response should behigh-pass and result in zero steady-state error. The disturbanceresponse governs the time the transmit power tracking loop 216 requiresto correct for gain error caused by amplifiers entering compression.Equation (13) defines the disturbance response. The variables inEquation (13) are provided in Table 1 and are described in more detailwith respect to FIG. 6. $\begin{matrix}{{{Disturbance}\quad {Response}} = {\frac{1}{1 + {ZDBK}}*G_{o}}} & (13)\end{matrix}$

The third response is the noise response. The noise response describesthe response of the gateway gain to the noise measurement of the gatewayoutput power. The noise response is roughly reciprocal of thedisturbance response. That is to say, the noise response is low-pass ifthe disturbance response is high-pass. Improved disturbance responsecomes at the expense of increased noise response. Equation (14) definesthe noise response. The variables in Equation (14) are provided in Table1 and are described in more detail with respect to FIG. 6.$\begin{matrix}{{{Noise}\quad {Response}} = {\frac{B_{Z}{KZD}}{1 + {ZDBK}}*N}} & (14)\end{matrix}$

The fourth response is the estimate response which describes theresponse of the gateway gain to noise in the measurement of the gatewayinput power. Again, the noise response is roughly the reciprocal of thedisturbance response. Therefore, the noise response is low-pass if thedisturbance response is high-pass and improved disturbance responseperformance comes at the expense of estimate response. Equation (15)defines the estimate response. The variables in Equation (15) areprovided in Table 1 and are described in more detail with respect toFIG. 6. $\begin{matrix}{{{Estimate}\quad {Response}} = {\frac{AKZD}{1 + {ZDBK}}*E}} & (15)\end{matrix}$

The fifth response is the command response. The command responsedescribes the response of the gateway to open loop commands. Thedynamics of the command response will govern the open loop control. As aresult, the command response should be designed to be as quick aspossible with little overshoot and zero steady state error. In thepreferred embodiment of the present invention, the open-loop commandscreate an overshoot in the gain of approximately 35% which reduces tozero after approximately one second. Equation (16) defines the commandresponse. The variables in Equation (16) are provided in Table 1 and aredescribed in more detail with respect to FIG. 6. $\begin{matrix}{{{Command}\quad {Response}} = {\frac{\left( {1 + {CK}} \right){ZD}}{1 + {ZDBK}}*G}} & (16)\end{matrix}$

The transmit power tracking loop gain (GAIN) 628 is the sum of the fiveresponses shown in Equations (12)-(16). Equation (17) for the transmitpower tracking loop gain (GAIN) is provided below. The variables inEquation (17) are provided in Table 1 and are described in more detailwith respect to FIG. 6. $\begin{matrix}\begin{matrix}{{GAIN} = \quad {{\frac{\left( {1 + K} \right){ZD}}{1 + {ZDBK}}*G} + {\frac{1}{1 + {ZDBK}}*G_{o}} + {\frac{\left( {A - B} \right){ZDK}}{1 + {ZDBK}}*}}} \\{\quad {{Pin} + {\frac{AKZD}{1 + {ZDBK}}*E} - {\frac{BzKZD}{1 + {ZDBK}}*N}}}\end{matrix} & (17)\end{matrix}$

The error signal filter gain 616 is primarily responsible for thetransmit power tracking loop 216 performance. In the preferredembodiment of the present invention, the error signal filter gain 616was selected to give zero steady state error and have a gain thatprovided a fast disturbance response. The digital filter applied to theerror signal 616 is defined as a continuous function of k in Equation(18).

K(s)=k/s  (18)

In one embodiment, the continuous value, k, was selected to be equal to2.38 (k=2.38).

As with the digital filters applied to the components of the transmitpower tracking loop 216 that provide power measurements, the value ofthe error signal filter gain 616 can be converted into a discrete valueby substituting s=(z−1)/T, where T is a 50 millisecond period of time.The discrete value for the digital filter applied to the error signal616 is K(z)=0.119/(z−1).

The discrete time filters are realized in the transmit power trackingloop 216 software as the following equations.

y(n)=linearized (d(n))

d(n)=k(n)+g(n)

k(n)=k(n−1)+0.199a(n−1)+0.119g(n−1)−0.119b(n−1)

b(n)=0.686b(n−1)+0.314p(n−1)

The variables of the equations above are defined as follows.

y(n)=the linearized sample output to the VGA D/A during time n, thelinearizer is realized as a lookup table

d(n)=intermediate value during time n

k(n)=intermediate value during time n

b(n)=intermediate value during time n

a(n)=the software estimate of total input power during time n

p(n)=the power measurement of total output power from the power meter236 during time n

g(n)=the open-loop command during time n

An additional design consideration is synchronization in measurementtime of power estimators 210 and power meter 236. In the preferredembodiment, synchronization is acceptable if it is no worse than 50 ms.

Other embodiments of the present invention are possible. The presentinvention is not limited to use in modulation systems for mobiletelephone systems. Any modulation system that benefits from control ofpower could include the transmit power tracking loop 216 of the presentinvention. In addition, the present invention could be used when thesignal is received as well as when the signal is transmitted.

In addition, measurement devices and measurements taken may be varied inthe present invention. The present invention may include additionalestimator summers 218 and/or may combine estimators 210 and estimatorsummers 218 in one unit. Similarly, multiple power meters 236 may beincluded in the present invention. Also, the present invention is notlimited in the use of a particular power meter 236, such as the HP437B.Any meter providing the functionality described may be used.Furthermore, additional power measurements may be taken by meters suchas power meter 236 or estimators such as estimator 210.

Other embodiments of the method of the present invention are possible.The present invention could be used to control the power of some of thenarrowband channels but not all. In addition, as mentioned, the presentinvention can be implemented with or without the narrowband channelshaving equal gains. If the gains of the narrowband channels are notequal, software correction can be used to compensate for theinequalities.

Other designs are possible in addition to the preferred design describedwith respect to the transmit power tracking loop input-output model 602.The filters described in the preferred embodiment of the presentinvention are not required, such as the estimate filter 222, the outputpower measurement digital filter 234, and the error signal digitalfilter 228. If one or more of these filters are implemented, differentvalues are possible other than those described. Also, additional filtersand amplifiers may be included in the system to improve the accuracy ofthe power.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, not limitation. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

What is claimed is:
 1. A method of controlling gain of a modulator,comprising: obtaining a transponder gain; obtaining an antennaelevation; calculating path gain using said antenna elevation;calculating a modulator gain using said path gain and said transpondergain; and adjusting the gain of the modulator.
 2. A system forcontrolling gain of a modulator, comprising: first obtaining means forobtaining a transponder gain; second obtaining means for obtaining anantenna elevation; first calculating means for calculating path gainusing said antenna elevation; second calculating means for calculating amodulator gain using said path gain and said transponder gain; andadjusting means for adjusting the gain of the modulator.
 3. A method ofcontrolling gain input into an antenna, comprising: obtaining an antennaelevation; calculating path gain using said antenna elevation; obtaininga transponder gain; obtaining an antenna dish gain; calculating a commongateway gain using said path gain, said transponder gain, and saidantenna dish gain; adjusting the gain of a variable gain amplifier.
 4. Asystem of controlling gain input into an antenna, comprising: firstobtaining means for obtaining an antenna elevation; first calculatingmeans for calculating path gain using said antenna elevation; secondobtaining means for obtaining a transponder gain; third obtaining meansfor obtaining an antenna dish gain; second calculating means forcalculating a common gateway gain using said path gain, said transpondergain, and said antenna dish gain; adjusting means for adjusting the gainof a variable gain amplifier.